Denitration or selective catalytic reduction (SCR) technologies are applied to combustion-derived flue gases for removal of nitrogen oxides. Denitration comprises reaction of nitrogen oxide species in the gases, such as nitrogen oxide (NO) or nitrogen dioxide (NO2), with a nitrogen containing reductant, such as ammonia or urea, resulting in the production of diatomic nitrogen (N2) and water.
Combustion flue-gases containing nitrogen oxides are commonly produced from the combustion of coal for electricity generation. Coal-fired combustion flue-gases contain high amounts of particulate matter, especially in the form of ash. This particulate matter has the ability to clog the cells of structural catalyst bodies resulting in a reduced catalytic performance and efficiency. Individual ash particles alone can plug catalyst cells or ash particles can aggregate to produce a plug. To address this problem, many utilities have modified flow control devices and honeycomb catalyst bodies to accommodate large particles in exhaust streams. For example, turning vanes and other fly ash capture apparatus have been redesigned to enhance ash capture efficiencies. Additionally, active cleaning mechanisms have been developed including soot blowers, sonic horn and pneumatic ash sweepers.
While being effective for mitigating effects of fly ash in the exhaust stream, these technologies insufficiently address the aggregation of fly ash on ductwork and beams upstream of catalyst modules in the reactor. Fly ash aggregate eventually breaks free from the ductwork and beams, striking inlet faces of catalyst bodies contained in the module. The volume of fly ash in an aggregate is substantial, often resembling an avalanche of material. Accordingly, significant catalyst plugging results.